I 2nd everything above about the peeing (possible UTI, possible small bladder and inability to hold it, and possible marking), but I wanted to add....
Like MauiGirl said, we don't know how old the pup is......if he's less than 1 yr-ish old, it could just be regression. Bella is 9 mo. old (which I guess is like a teenager in dog terms) and for some reason after Xmas, she decided that she didn't really have to go on the pad anymore
Man, did that get frustrating! I felt like I had to go back to stage one to re-train her, but she picked it all back up much faster this time around, thank goodness!! Anyway, I've heard that even when they are 100% trained, pre 1 yr-ish there is always the possibility of regression and having to re-train and be firm!
(Do check about the UTI, though, if it continues. That would be my first thing to mark off the "maybe it's...." list because UTI's are at the very least uncomfortable if not painful, AND letting one go too long can allow bladder crystals or stones to form, which you don't want!!)